The present invention relates to a process for the production of aluminum alloy castings, especially of Al--Si and Al--Si--Cu alloy castings.
Conventionally, Al--Si alloy castngs having high toughness are produced from metals with minimum contents of impurities, especially of iron. Impurities present in metals cannot be removed without refining. Iron, which is one of the most problematic impurities that are easily introduced into metals, forms needles of an Al--Fe--Si compound, which not only reduces the toughness of the casting but also induces structural defects therein. Impurity iron is also introduced into the casting as a carryover from secondary metals (partially refined metals), scrap and return scraps. A method is known to alleviate the adverse effects of iron and provide a higher toughness by adding a suitable amount of manganese in order to crystallize the iron not as needles but as irregular-shaped particles of an Al--(Fe,Mn)--Si compound. However, even this method is not completely effective in eliminating the adverse effects of iron, and manganese added in an amount exceeding a certain level will impair the castability of the metal.
Aluminum alloy castings are subjected to a variety of thermal treatments. Among them is solution heat treatment wherein the alloy is heated to a high temperature to dissolve as much solute as practicable for ensuring a maximum result in the subsequent age hardening. Solution heat treatment also has the advantage of forming spherical eutectic Si particles in Al--Si alloys so as to increase their toughness.
While solution heat treatment is desirably effected at the highest possible temperature for the longest feasible period, it has been generally understood that in order to avoid "burning" that leads to a reduced strength, the temperature should not exceed the final solidification point of the casting in the nonequilibrium state. The period of solution heat treatment is also limited from cost and productivity viewpoints. As a result, not all of the solutes are put into solution in the matrix (solid solution of elements in aluminum) and some are left as crystals, and the eutectic Si particles are far from being spherical. Because of these facts, aluminum alloy castings are currently used without making the most of the inherent characteristics of the alloy.